A variety of metal-containing compositions (i.e., pastes, inks, tapes, etc.) useful in forming resistors, dielectrics and conductors which are employed in hybrid-micro electronic components have been developed in the field of electronics. Generally, such compositions, and particularly a paste or an ink composition, include a conductor (e.g., copper, aluminum, gold, silver, platinum, tin, and the like as well as alloys of each of these different metals), resistive or dielectric components, a binder, (e.g., a glass or inorganic oxides), and a carrier comprising generally a solvent with a resin and a thixotrope and/or a wetting agent.
The above-described paste or ink compositions are applied in the desired configuration or pattern onto a suitable substrate to form the desired circuit for use as a microelectronic component. There have been a number of substrate materials developed for use in these applications. For example, such substrate materials may include alumina (Al.sub.2 O.sub.3), beryllia (BeO), aluminum nitride (AlN) and silicon carbide (SiC).
Until recently, most circuit substrates have been made of ceramic substrates such as alumina (Al.sub.2 O.sub.3) or a resin substrate. In some applications, Al.sub.2 O.sub.3 substrates have superior mechanical strength and electrical insulation properties. Additionally, such substrates can be easily made into green sheets, which are utilized in the production of high-density multilayer hybrid circuits. Thus, Al.sub.2 O.sub.3 has found wide or extensive use. However, the thermal conductivity of Al.sub.2 O.sub.3 is low, only about 20 W/mK.
In recent years, as electronic devices have become smaller, the packaging density of electronic devices (such as IC's) mounted on a circuit substrate has increased. In addition, power semiconductors are being used. Consequently, a great deal of heat is produced by the electronic devices, making it necessary for the circuit substrate to radiate heat away efficiently. However, since the thermal conductivity of Al.sub.2 O.sub.3 is low, when a great deal of heat is produced, it is impossible to expect much of it to be radiated away from the circuit substrate. Consequently, when mounting electronic devices in a high-density packaging configuration or producing modules containing power semiconductors, it is desirable to have a circuit substrate which has high thermal conductivity in addition to mechanical strength and good electrical insulation.
In recent years, progress has been made in fine ceramics technology. For example, ceramic materials such as silicon carbide and aluminum nitride have been developed which exhibit superior mechanical strength and thermal conductivity. While much research has been conducted with silicon carbide, this material has a high dielectric constant and low dielectric strength and, in turn, presents serious problems in using it for high-frequency circuit devices and for devices to which high voltage will be applied.
Aluminum nitride on the other hand exhibits good electric insulation as well as thermal conductivity and thus has been found promising for application as circuit substrates. This material has thermal conductivity in the range of at least about 100 W/mK, mechanical strength of about 40 to 50 Kg/mm.sup.2 and a dielectric strength of about 140 to 170 kV/cm. Thus, aluminum nitride, due to its high thermal conductivity, is an attractive substrate material to the power hybrid market. However, because its surface is not an oxide, or contains very little Al.sub.2 O.sub.3, conductive compositions designed for alumina have nothing to react with and thus a somewhat weak bond is generally formed. Also, there can be a problem of blister formation during the processing of most circuit substrates of this nature. Specifically, when the glass binder used in conductive compositions begins to react or flood over the aluminum nitride, nitrogen gas is liberated causing blisters to be created in the film, thus lifting the film off of the substrate.
Of the possible metals that may be used as conductors in these conductive compositions, copper (Cu) is considered optimal for a number of reasons. Copper offers the signal speed of silver while avoiding the problems of the leaching or migrating of silver or other noble metals. However, a copper conductive composition for use on ceramic substrates comprised of aluminum nitride has not yet been developed.
Aluminum nitride substrates for use in microelectronic devices has been described in the art, for example, Norton, in the Journal of Material Science Letters, 9 (1990) 91-93, describes a lithium-based paste composition which is screen printable on alum nitride substrates and is thermodynamically stable.
U.S. Pat. No. 4,540,673 describes semi-conductor devices using a sintered aluminum nitride having high thermal conductivity and containing at least one other metal compound selected from beryllium (Be), lithium (Li) and compounds of these metals.
United Kingdom Patent specification 1,251,766 discloses a noble metal metallizing composition that may be applied to glass substrates and fired to produce a conductive metallic coating.
U.S. Pat. No. 4,400,214 discloses a conductive paste comprising particulate copper, aluminum (Al), and at least one of zinc (Zn) a nd silver (Ag) for use in making conductors for microcircuit conductors. More specifically, alloy powders useful for making up a conductive paste to be applied to the appropriate substrate are disclosed.
A stain-resistant ruthenium oxide based resistor composition is disclosed in U.S. Pat. No. 4,476,039. The ruthenium oxide based compound containing composition is useful as a stain-free printable thick film resistor composition for microcircuit applications.
U.S. Pat. No. 4,251,397 discloses vehicles useful for thick film resistors that are fireable in nonoxidizing atmospheres. Specifically, it is disclosed that film paste based on compositions where the vehicle or carrier is based on copolymers of ethylene and vinyl acetate have high viscosity, good printability and exhibit no carbonaceous residue upon firing.
Glass binder compositions for thick film paste compositions are disclosed in European Patent application 0 132 810. It is disclosed that a borosilicate glass composition free of bismuth, cadmium and lead is especially useful as a binder for thick film resistor compositions and also such a binder serves to adjust the TCR of resistors made therefrom.
European Patent application 0 153 737 discloses a high thermal conductivity circuit substrate that comprises sintered aluminum nitride ceramic which further consists essentially of at least one member selected from the group of yttrium, the rare earth metals and the alkali earth metals and an electrically conductive thick film paste applicable for forming a conductive layer on the substrate.
None of the foregoing references suggest or describe a copper conductor composition which is applicable for aluminum nitride ceramic substrates. Moreover, none of the foregoing references suggest or describe a microelectronic circuit substrate having a copper conductive composition fired on an aluminum nitride substrate.